About Genders
Persons have biological genders. This is independent of any language, and there are only two biological genders (as long as we aren't talking about fungi).
Words in languages can have any amount of genders, depending on the language. English nouns, articles and adjectives all have the same gender, so gender is not a category in English language. Only personal pronouns can have different genders in English, but this is not enough to classify English to be a multi-gender-language. English is a single-gender-language with gender-pronouns.
- Finnish also is a single-gender-language, and it even has no gender-pronouns (Finnish "hän" means both: he and she).
- Italian has two genders (the latin names for those genders are "masculum" and "feminam", meaning "male" and "female").
- Swedish also has two ("utrum", meaning "both" which is a combination of masculum and feminam, and "neutrum", meaning "non of both").
- German has three genders: "masculum", "feminam" and "neutrum". You call them »männlich«, »weiblich« and »sächlich« in German, or in Engish: "male", "female" and "neuter".
- All European languages have between one and three genders out of a set of four possible genders: "masculum", "feminam", "utrum" and "neutrum"
But there are even languages like the African language Swahili that have more than 20 different genders. In this case you no longer call this feature »genders« but »noun classes«, but technically it's the same. And this is what you should have in mind when talking about grammatical genders: They are noun classes, i.e. grammatical categories for nouns. And in many languages, among them German, other parts of speech, that are somehow bound to nouns, inherit the nouns class (gender) and also appear in that nouns class, although they are not nouns themselves.
And this is what articles, adjectives and pronouns do in German: They inherit the gender of the noun to which they refer. This is not true for English: the personal pronouns he, she and it do not inherit any grammatical property from any other word!
I repeat, because this a very important difference between German and English:
In German pronouns, articles and adjectives inherit the grammatical gender of the referred noun. In English the personal pronouns he/she/it do not inherit any grammatical property from any other word.
In English those pronouns reflect the biological gender of the person to which the pronoun refers, but only if the pronoun really refers to a person (or a pet, an android or something else that is somehow person-like). If it refers to anything else but a person, then it has to be it.
Also note, that it's not the things named by words that have a grammatical gender. It's the word itself that carries the gender. If you use different words for the same thing (i.e. synonyms), they often have different genders. The very same shabby old limousine can be in German
- das Auto (neuter) (The word Auto can be used for all cars)
- der Wagen (male) (A Wagen is a big limousine)
- die Karre (female) (A Karre is a shabby car)
Your Sentence
Let's take a look on your sentence:
Ich denke, dass es eine sehr interessante Festlichkeit ist, weil es einzigartig ist.
I think, that it is a very interesting festivity, because it is unique.
Both pronouns in each sentence (in German both instances of es, as well as in English both instances of it) refer to the same noun. In English this could be festivity, because festivity is not a person, and therefore the pronoun it might match with it.
But this doesn't work in German. Any noun ending with -keit is female. So a pronoun that would match with Festlichkeit must be female to, i.e. it had to be »sie«:
Ich denke, dass sie eine sehr interessante Festlichkeit ist, weil sie einzigartig ist.
But the truth is, neither in the English sentence, nor in the German, the two pronouns are referring to Festlichkeit or festivity. In both cases, they are referring to a noun outside the sentence. They refer to a noun in the context, that you did not tell us in your question.
This word could be feast or party or birthday (or something else). Note, that this three words translate to German words, that have three different genders:
- feast = das Fest (neuter)
- party = die Party (female)
- birthday = der Geburtstag (male)
Context
So, let's add some context, and let's look at the sentences then:
the feast = das Fest (neuter)
The king's feast is celebrated in the castle.
I think, that it is a very interesting festivity, because it is unique.
Im Schloss wird das Fest des Königs gefeiert.
Ich denke, dass es eine sehr interessante Festlichkeit ist, weil es einzigartig ist.
the party = die Party (female)
Lisa is celebrating the party of the year.
I think, that it is a very interesting festivity, because it is unique.
Lisa feiert die Party des Jahres.
Ich denke, dass sie eine sehr interessante Festlichkeit ist, weil sie einzigartig ist.
the birthday = der Geburtstag (male)
Family Stein is celebrating the birthday of their youngest son.
I think, that it is a very interesting festivity, because it is unique.
Familie Stein feiert den Geburtstag ihres jüngsten Sohnes.
Ich denke, dass er eine sehr interessante Festlichkeit ist, weil er einzigartig ist.
So, your question can not be answered without context, because the gender of the pronoun depends on the gender of a noun in this context.
»Sie« (capitalized) vs. »sie« (not capitalized)
You also asked, »Or should I change the pronoun "Es" for the pronoun "Sie"?«, and an this question you wrote »Sie« capitalized.
Note, that »sie« and »Sie« are different pronouns!
- sie = she (personal pronoun 3rd person singular)
or
- sie = they (personal pronoun 3rd person plural)
but
- Sie = you (personal pronoun 2nd person, polite form, both: singular and plural)
In your sentence the pronoun is not referring to the person to which you speak, so it is not 2nd person. It also is not referring to the speaker (which would mean 1st person). It is referring to something that is neither the sender nor the receiver of the message, and this is per definition is 3rd person. So you can't use Sie. You have to use sie (or er or es, depending on the gender of the referred noun).